If you start off dotting when you first enter Solar eclipse. With decent enough haste and DI you will have time to do 3 casts "past ending solar" and then refresh. You could get 4 casts before redotting depending on how far away you are from ragnaros but then I've noticed that NG sometimes drops off so better standing alilttle bit closer.
After redotting them just before leaving Solar you can easily enter Lunar and redot when theyre about to expire, spam starfire and then youre back to step1.
Now if you don't have NG the second time to redot in Solar, you're doing something wrong or have had to move very much (Traps spawns etc), in which case I would redot without NG since you'll basicly only have ~3 secs left on the dots as you refresh. If you dont have NG you have time to do 2 wrath before having to refresing dots (again, 3 if you on the far edge of ragnaros platform)
It's pretty easy when you get into it. Notice: This is only for ragnaros, I wouldnt recommend casting 3 wraths before leaving solar to redot on random other encounter.